If you do not remember how a FET works, refer to NEETS Module 7 Introduction to Solid-State
Devices and Power Supplies.
The third single-stage audio amplifier is shown in figure 1-25. This is a class A, common-emitter,
transformer-coupled, transistor, audio amplifier. The output device (speaker) is shown connected to the
secondary winding of the transformer. C1 is a coupling capacitor which couples the input signal to the
base of Q1. R1 develops the input signal. R2 is used to bias the emitter of Q1 and provides temperature
stability. C2 is a decoupling capacitor for R2. R3 is used to bias the base of Q1. The primary of T1 is the
collector load for Q1 and develops the output signal. T1 couples the output signal to the speaker and
provides impedance matching between the output impedance of the transistor (medium) and the
impedance of the speaker (low).
Figure 1-25.Single-stage audio amplifier.
Sometimes it is necessary to provide two signals that are equal in amplitude but 180º out of phase
with each other. (You will see one use of these two signals a little later in this chapter.) The two signals
can be provided from a single input signal by the use of a PHASE SPLITTER. A phase splitter is a device
that produces two signals that differ in phase from each other from a single input signal. Figure 1-26 is a
block diagram of a phase splitter.
Figure 1-26.Block diagram of a phase splitter.
One way in which a phase splitter can be made is to use a center-tapped transformer. As you may
remember from your study of transformers, when the transformer secondary winding is center-tapped,
two equal amplitude signals are produced. These signals will be 180º out of phase with each other. So a
transformer with a center-tapped secondary fulfills the definition of a phase splitter.